1. Field of the Invention
The invention lies in the field of optical signal processing systems. More specifically, the invention relates to the address field recognition in an optical packet-switched network.
2. The Prior Art
In an optical packet-switched network, data, often in digital form, are transported through the network in optical packets. An optical packet is an optical signal, modulated with data, of a certain frequency and having a certain defined structure. It is customary that such a packet, such as, for example, an ATM cell, comprises a header section and a data portion, hereinafter referred to as address field ("header") and information field ("payload") respectively. The address field comprises, among others, coded routing and/or destination information, while the information field contains the actual data which are to be transported through the network. Once the packet arrives in a node of the network, that packet must, on the basis of the information in the address field of the packet, be switched through to either a receiver connected to the node or to a following node. The address field of the packet must thereto be read and possibly altered for transport to a following node, while the information field remains (or can remain) unread and unaltered. In principle, the optical packets from the optical domain can, for that purpose, first be converted to the electrical domain; in the electrical domain, after analysis and possibly modification of the address field, the packets can then be switched; and subsequently, the switched packets can again be converted to the optical domain. In light of the desire for ever higher bit speeds, however, this switching procedure is much too slow. Another possibility is to leave the optical packets in the optical domain as much as possible, and to switch optically. The optical switching means which need to be applied thereto, however, require an electrical driver. For that reason, a relatively small portion (for example, 10%) of the optical power which the optical packet contains is split off in order to derive therefrom, after analysis in the optical or electrical domain of the address and routing information in the address field, electrical control signals for driving the optical switching means. If necessary, the optical packet (that is, the remaining portion of the signal power, which contains the full packet information) is meanwhile, via temporary storage means such as, for example, a delay line, conducted to an input of the switching means.
A technique in which the address filed analysis is carried out in the electrical domain in known, for example, from reference 1! (for further bibliographical data relating to the references quoted, see hereinafter under section C). In this case, the split-off portion of the packet signal containing the full packet information is completely converted into an electrical signal. Since the length of the address field signal is in general much shorter than the information field signal (for an ATM cell, for example, the length ratio of the address field and information field signal is approx. 1:10), in fact an unnecessarily long time occurs before the actual analysis can start.
From reference 2!, a technique is known in which, of the split-off portion of the optical signal of an ATM cell, with the aid of a synchronization signal obtained simultaneously from the split-off portion, only the first 5 bytes (the address field) are converted into an electrical signal, i.e., an "electrical address field". From analysis of the electrical address field signal, a driver signal is derived for the switching means. The analysis also results in a new address field signal which is converted into an optical signal. Via a beam coupler, the new address field signal is coupled synchronously to the other portion of the ATM cell signal, in order to replace the old address field signal therein.
From reference 3!, a network is known in which from the split-off portion of the optical packet signal, in the optical domain, the address field signal is separated. This reference describes, an optical telecommunication system in which transmitted packets are composed of an optical data signal of a first wavelength, modulated with the data to be transmitted, and an optical control signal of a second wavelength. The second wavelength is specific for a certain destination, and therefore in fact forms the address field signal of the packet. A network node of the system is provided with an optical switch and with control means which selectively react to signals of the second wavelength for the routing of a related packet by the switch. In that case, a portion of the signal power of a packet is split off and examined by filtering in regard to a signal of the second wavelength which is specific for the node. When the node-specific signal is detected, the signal is converted into the electrical domain and applied as control signal for the switch. The system known from reference 3! has a number of limitations. As a result of the application of signals of various wavelengths within one packet, extra measures are required to counteract dispersion problems. The wavelength range from which the node-specific wavelengths are chosen is no longer available for a possible extension of the transmission capacity of the network. No possibility is indicated of giving the packet another destination while it is under way.
Besides the limitations previously noted above, the known techniques discussed above further have the disadvantage that, due to the splitting off of the signal for the purpose of analyzing the address field of the packet, a portion of the packet signal power is lost. In the technique of reference 2!, such a loss of signal also occurs in the beam coupler to which the new address field signal is synchronously coupled for the purpose of replacing the old address field in the packet signal. Such signal losses demand additional signal amplification, especially if packets are to be routed via various nodes.
From references 12! and 13!, an inter-connection technique for a multiple stage optical switching network is known in which transmission of data and address information takes place by means of optical signals which propagate in free space, and which are mutually orthogonally polarised. For switching purposes an address signal, at any rate a portion of the power of an address signal, is separated from the corresponding data signal in a so-called partial polarization beam splitter (PPBS). This address signal is compared, in the electrical domain, with a reference address signal, which can be an address signal of another data signal. Based on the result of the comparison, the related data signal, with the thereto preceding address signal (with reduced power), is switched through. As a great advantage of the application of mutually orthogonal polarization states for the address signal and the data signal, these references mention that these signals can be separated in a straightforward manner by application of passive optical devices. However, a restriction of this technique is that a good optical separation on the basis of polarization requires that the polarisation beam splitters continually have the proper orientation with respect to the polarisations of the signals to be separated. In inter-connection applications this is not a problem, since the distances to be bridged are often in the order of centimeters. In optical packet-switched networks, such as those applied for telecommunication purposes, and in which optical signal transmission takes place over distances which are more likely in the order of kilometers, and via transmission lines such as glass fiber connections which are usually not polarization-maintaining, this technique can not be utilized without further measures.